Poisson’s (Arago’s) Spot with Office Supplies

1. SUNY Buffalo State College Physics Science Building 262Buffalo State College1300 Elmwood AvenueBuffalo, NY 14222physicsEURP09@gmail.com http://www.physicsed.buffalostate.edu/EURP09 Abstract: A green laser beam is expanded with some lenses and the light bends around a map pin or lead pellet. Light interacts with the edges of the head of the pin which creates a diffraction pattern. The pattern includes internal rings and a bright spot in the center of the shadow of the disk. Other small objects can replace the map pin for different diffraction patterns. Apparatus Competition2009 AAPT Summer MeetingAnn Arbor, MichiganAlyssa Cederman, Jessie Segal,and Dan MacIsaac Poisson’s (Arago’s) Spot with Office Supplies • Construction of Apparatus: • Attach two binder clips to a meter stick, then flip the meter stick • Put a binder clip on the switch of a laser and put them on a stack of magnets • Use a lens to expand the beam to the final desired size for projection. Here we used two -20cm lenses (they were at hand) in binder clips that are 19cm and 26cm from the laser • Stick a map pin in a binder clip and stick it on a magnet 35cm from the laser • Align apparatus on the meter stick by using double-sided foam mounting tape and binder clips and project image on screen Use of Apparatus: This apparatus can be used to view Poisson’s (Arago’s) Spot and other diffraction patterns with cheap, common, sturdy, and replaceable apparatus. Poisson’s Spot proves that light has wave-like properties.

2. Tips:Map pins can be used to create Poisson’s (Arago’s) Spot. However, the metal stick can interfere with the diffraction patterns. To get a very clear image of Poisson’s (Aago’s) Spot, you can replace the ball of a map pin by taping two microscope slides together and insert a size 9 lead shot (pellet). Notes: • We used two lenses to expand the laser beam because the lenses are not very strong and if stronger lenses were used, only one lens would be necessary; diverging lenses are held more tightly by the binder clips than converging ones • The lens’ purpose is to expand the laser beam to be large enough to surround the object and to project the final image • A green laser is preferred because our eyes are much more sensitive to green light – green appears MUCH brighter than red • Magnets are used because they are readily adjustable and available – they attract binder clips and elevate the apparatus • Binder clips are used to mark locations on the meter stick, to stabilize the meter stick, to hold down the switch of the laser, and also to hold apparatus in place History of Poisson’s (Arago’s) Spot: In an attempt to disprove wave theory of light, Siméon-Denis Poisson predicted, using Augustin Fresnel’s theory, that light should bend around a small sphere and the light should interfere to create a bright spot superimposed on the center of the sphere’s shadow. Dominique Arago experimentally proved Poisson’s prediction to be true. Thus it is now called both Poisson’s and Arago’s Spot. Equipment List: